The present invention relates generally to a method and apparatus for compressors, and more particularly to a method and apparatus for controlling compressors in parallel or in series which enables all of the load-sharing compressors to carry their optimum share of the load.
Conventional control systems of compressor station consisting of centrifugal and/or axial compressors do not cope with two major problems:
Those associated with load sharing of multiple compressors working in parallel and/or in series and those associated with controller loop interaction especially between process control loops and antisurge protective loops.
Conventional load-sharing techniques for multiple compressors allow one compressor to operate closer to its surge limit than other compressors. For instance for parallel operations, one compressor may even be overloaded to the risk of tripping off and shutting down, while another compressor is less loaded on the verge of going into surge. This creates not only the dangers of surge-and overload-created compressor damage, but also of wasted energy through recycling or blowing off flow and the possibility of process interruption.
Since conventional load-sharing control strategies are unable to load compressors properly, compressors frequently wrestle loads back and forth between them. This load oscillation degrades system performance and efficiency, and it may lead to damage or process shutdown.
The normal function of a station process control is to maintain its controlled variable equal to some set point. For instance, it may be required to maintain the discharge pressure or flow of the station. When called upon to protect one of the compressors from surge, an antisurge controller will have, as its function, to reduce the compressors' pressure differential and to increase flow at the same time. Operating independently and at cross purposes, then, conventional process controllers and antisurge controllers may create oscillations of pressure and flow that degrade process control and reduce antisurge protection.
To prevent such oscillations, conventional process and antisurge controllers are made to react slowly. This results in degraded process control, reduced antisurge protection and surge damage because of the slow control response and because of interaction between the two controllers. It may also shorten compressor life due to surges resulting from less than optimum control. Conventional controllers cannot achieve fast control and stability at the same time. They, therefore, are detuned to provide sluggish, but stable, control.
The control of multicompressors may be significantly improved by using a cascade control including a process control loop, load-sharing control loops and antisurge protective loops. The load-sharing loops enable all of the load-sharing compressors to carry their optimum share of total flow (for parallel operations) or total pressure differential (for series operation), providing for equidistant operation from surge control lines. In addition, each load-sharing loop provides for effective decoupling between process control loop and control member of compressor while the compressors' operating point crosses the surge control line.